Recently, as mobile communication industries and information electronic industries develop rapidly, a light-weight, high-capacity lithium secondary battery is increasingly in demand. In particular, a lithium secondary battery has the advantage of a higher drive voltage and a higher energy density compared to other conventional batteries. However, a lithium secondary battery that uses an organic electrolyte may ignite and explode, and thus a lithium secondary battery that meets with sufficient levels of safety becomes the focus of intention. To provide against overcharge or overdischarge occurring away from the normal drive condition of a lithium secondary battery, a protection circuit has been attached to a lithium secondary battery. However, protection circuits, PTCs or thermal fuses, attached to a lithium secondary battery for the purpose of safety, are not preferable. This is because they are expensive and take up large volume. Therefore, a battery having no protection circuit is very much in demand.
Meanwhile, the bare cells that have no protection circuits developed up to date, have a problem. They show a rapid drop in the capacity when they are subjected to a charge/discharge cycle again after an overcharge test. Moreover, when such bare cells are overdisharged to a voltage lower than an adequate voltage, they are not amenable to charge/discharge cycles any longer due to their remarkably decreased capacity in spite of an attempt to recharge them.
In the above case, rapid decrease in the capacity after overcharge results from the following reason. In general, voltage of a battery is defined by the potential difference between a cathode and an anode. When a battery is discharged continuously at low electric current after its voltage drops to a level lower than the normal voltage, the cathode voltage decreases gradually due to the consumption of Li ions in the anode. On the contrary, the anode voltage rapidly increases in advance of the cathode voltage, and finally reaches a voltage of 3.6V, where the copper foil used as an anode collector is oxidized. Thus, the copper foil is dissolved out in the form of copper ions, and contaminates the electrolyte. Then, the copper ions are reattached to the anode surface during the next charge cycle, and thus the anode active material is not utilizable any longer. Therefore, oxidation of copper foil results in a rapid drop in the capacity of a battery after overdischarge, and thus makes the battery useless.
To solve the above-mentioned problem, many attempts were made to inhibit the increase in anode voltage by decreasing cathode potential according to the prior art. For example, the SONY Corporation developed a method for preventing Cu in the copper foil used as an anode collector from reaching the potential causing the corrosion of Cu. The SONY corporation's method comprised incorporating a material with a low discharge voltage (i.e. a material capable of decreasing the cathode voltage rapidly), such as olivine or a nickel-containing material, into the cathode active material, so that the cathode voltage decreased rapidly at the last stage of discharge to complete discharge as early as possible. However, such a change in the cathode active material caused degradation of battery quality, such as gas formation at high temperature. Therefore, development of a novel method for preventing dissolution of Cu while maintaining the overall quality of a battery is required.